DWYWAI (standardized modern form)
The poem Y Gododdin includes a reference to "the son of Dwywai" in a context where it has been interpreted as a metronym for the poet Aneirin (Williams 1990, Koch 1997). This Dwywai has also been associated with Dwywai, daughter of Lleenog, mother of Saint Deiniol, and sister of Gwallog the king of Elmet. On a chronological basis, this equation is certainly plausible, with both women having a calculated date ca. 500. Whether or not it is historically true, both women clearly have northern associations.
Dwywei -- the B1 text of the Book of Aneirin (while the text itself was probably composed around the 7th century, the spellings reflect a more recent revision, perhaps as late as the 11th century)
Dwywei -- Bonedd y Seint (Bartrum EWGT), mss. dating to the 13th c. and later
Koch (1997) discusses this name in detail, deriving it from a Brittonic root meaning "god", although he notes that the name could have arisen either in the context of a pre-Christian tradition (from *Deweia "Goddess-like") or a Christian tradition (with a meaning along the lines of "belonging to God"). He reconstructs a Primitive Welsh form Deue, representing a pronunciation ['de-we].
This would suggest a plausible 6th c. Latinized written form along the lines of Deuea. In English syllables, the pronunciation would be something like "DAY-way".